Many devices, due to their operation and throughput, produce heat which accumulates and adversely affect their continuous performance unless conducted away and dissipated. This is particular true for semiconductor devices such as processor devices (where the ever-increasing VLSI and processing speed and amount of data bits processed), liquid crystal displays(LCD), illuminating devices such as light-emitting diodes (LED), etc. where various heat transfer devices are being employed for thermal management.
One of the significant advances made is in respect of heat pipes which may be employed in a flexible structure comprising multiple laminates such as disclosed in U.S. Pat. No. 6,446,706 (Thermal Corp.) as shown in FIGS. 1a and 1b (Prior art). The flexible heat pipe includes a sealed outer casing (26) comprising a polypropylene layer (28), a first metal foil layer (32) attached to the polypropylene layer (28) by a first adhesive layer (30), a second metal foil layer (12) attached to the first metal foil layer 32 by a second adhesive layer 34, and a wick layer 24 which is formed using a flexible and porous material.
The heat pipe further includes a separation layer 18 which supports the wick layer 24 such that the wick layer 24 stays in close contact with the outer casing 26 and allows vapour to flow in many directions in the casing. The separation layer 18 is realized as a mesh screen made of polypropylene. The wick layer 24 is made of a copper felt material. The copper felt comprises micro-fibres, each having a diameter of 20 micro inches and a length of 0.2 inches, and copper powder filled in the wick structure in an amount of 20 to 60% of the total volume of the wick structure.
Whilst the flexibility of the laminated layers allows it to be affixed over and conforms to a device to be cooled, contact surfaces between the various laminate layers and the flexible heat pipe may be affected by the flexible material and configuration, thus affecting effective heat conduction.
FIG. 2 (Prior Art) illustrates a plate-type heat transfer device according to Korean Patent Laid-Open Publication Number 10-2004-18107. The heat transfer device comprises an upper plate 200, and a lower plate 100 disposed under the upper plate 200, having a gap between the upper plate 200 and the lower plate 100, in which the lower surface of the lower plate 100 corresponds to an evaporation part P1 and is in contact with a heat source. The heat transfer device further comprises wick plates 120 disposed so as to be in close contact with the upper surface of the lower plate 100 due to the surface tension of liquid coolant, and a spacer plate 110 for maintaining the distance between the lower plate 100 and the wick plate 120.
The liquid coolant circulates between the evaporation part P1 and a condensation part P2. That is, the liquid phase coolant continuously flows to the evaporation part P1 by means of capillary force generated between it and the lower plate, enters a vapour phase at the evaporation part P1, flows in a vapour phase toward the condensation part P2, and condenses at the condensation part P2. The spacer plate 110 serves to maintain the distance between the lower plate 100 and the wick plate 120 by using the surface tension generated between of them.
FIG. 3 is an illustration of third prior art of a flat sheet type heat transfer device disclosed in Korean Unexamined Patent Application No. 10-2004-91617. The heat transfer device shown in FIG. 3 comprises an upper metal plate 300, a lower metal plate 350, a pressuring support structure 310, and a plurality of thin plates 320 and 322, the pressuring pressure tension structure 310 and the thin plates 320 and 322 being interposed between the upper plate 300 and the lower plate 350. Each of the thin plates has through patterns that are parallel to each other, formed by a micromachining process. The pressuring pressure tension structure 310 is made of a porous material such as a mesh screen having through holes dense enough so that vapour, generated by the vaporization of coolant, occurring because the heat source is in contact with the lower surface of the lower plate 350, can move in a vertical direction.
The pressuring pressure tension structure 310 presses at least a portion of the parallel patterns of the thin plates 320 and 322 when assembled. Due to the pressure from the pressuring support plate 310, the parallel patterns of the thin plates 320 and 322 are form close contact with the upper surface of the lower plate 350, so that micro gaps, smaller than those of the patterns in an initial state, are formed. The micro gaps form fine coolant passages that are of few micro meters which are difficult to realize by the processing method such as etching or machining.
There are several limitations associated with the first prior art(U.S. Pat. No. 6,446,706). Firstly it is difficult to make the heat pipe which has a complex inner structure. Since the wick layer 24 is made of copper felt it is very difficult to maintain regular and strong contact between the inner surfaces of the outer casing and the wick layer 24. As such, forming of micro paths in the wick layer 24 is irregular, causing non-uniform capillary force that drives the flow. This creates high flow resistance which causes weak capillary force. Accordingly, when the coolant actively evaporates around a heat source, the flow of the vapour phase coolant may be cut off. Moreover, heat conductivity varies from point to point. Thus, reproducibility of the heat transfer device is poor. Another limitation is the thinness of the copper felt. Due to difficulty in manufacturing thin copper felt the total thinness of the heat pipe is limited by the thickness of the copper felt.
The second prior art(Korean Patent Laid-Open Publication Number 10-2004-18107) has also limitations. Micro machining is needed to manufacture a thin and complex structure to be inserted between an upper plate and a lower plate, thus limiting mass production. Accordingly, the device's enclosure can be manufactured no thinner than several mm thick. The device's configuration is structured according to the liquid coolant flows in gaps formed between planar wicks provided in the wick plate 120, or gaps formed between the wick plate 120 and the lower plate. Since the device incorporates micro structures, such as bridges, for connecting protrusions formed on the lower plate and the upper plate or connecting planar wicks, in order to form uniform gaps and to be mounted in the device confined enclosure, it is difficult to precisely machine such micro structures, as the micro structures are so complex and are several millimeters thick. Also, non-uniform gaps can result in drying out of the liquid phase coolant at the evaporation part, thereby causing fatal failure of the heat transfer device. In particular, mass production of such micro structures is more difficult since the structure is so much complex and machining errors can occur.
The third prior art(Korean Unexamined Patent Application No. 10-2004-91617) has following limitations. As shown in FIG. 4a and FIG. 4b, the thin metal plate or mesh is not wettable or liquid-absorbing because of the nature of the material and its design. This can create repelling of coolant that can cause dry out phenomena to occur. Furthermore, maintaining fine passages are very difficult due to manufacturing difficulties, increasing the cost of manufacturing. Reducing the thickness of the metal plate or mesh is critical in reducing the thickness of the device and the electronic device this is applied, but this process is difficult and incurs extra cost.
It would therefore be ideal to have a heat transfer device that overcomes the above limitations and disadvantages, towards which it is now proposed, as a general embodiment, a heat transfer device comprising at least an aggregate of fibres or sheet of fibres with internal passages and holes capable of capillary transport of liquids from a heat source region to heat dissipation region and vice versa; a supply of coolant fluid in sufficient amount absorbed or adsorbed by said fibres or sheet of fibres with internal passages and holes capable of capillary transport of liquids; pressure tension member (32) comprising a strong yet resilient structure placed within said confined space and exerting pressure on said aggregate of fibres or sheet of fibres with internal passages and holes capable of capillary transport of liquids against said heat source region (30) and/or heat dissipation region, wherein a plurality of undulations are provided on said pressure tension member; and a casing enclosing hermetically the aforesaid in a confined space.